Plants have a number of means to cope with nutrient deficiencies, such as poor nitrogen availability. They constantly sense nitrogen availability in the soil and respond accordingly by modulating gene expression. Although more is being discovered about nitrogen and the components involved in regulating its uptake and use, much is still unknown about many of these complex interactions. For this reason, it is interesting when a gene of known or unknown function is shown to have a nitrogen response, as it opens up new possibilities and insights into nitrogen use and nitrogen use efficiency in a competitive environment (i.e. low and/or high nitrogen).
Nitrogen regulated gene expression is an important aspect of a plant's response to changes in nitrogen availability. Nitrate acts as a signal to initiate a number of responses that serve to reprogram plant metabolism, physiology and development (Redinbaugh and Campbell, 1991; Forde, 2002). Nitrogen-inducible gene expression has been characterized for a number of genes in some detail. These include nitrate reductase, nitrite reductase, 6-phosphoglucante dehydrogenase, and nitrate and ammonium transporters (Redinbaugh and Campbell, 1991; Huber et al., 1994; Hwang et al., 1997; Redinbaugh and Campbell, 1998; Gazzarrini et al., 1999; Glass et al., 2002; Okamoto et al., 2003). Investigations into the cis acting control elements and DNA binding factors involved in nitrate regulated gene expression have focused on the nitrate reductase gene from tobacco and spinach and have identified several putative regulatory elements (Rastogi et al., 1993; Lin et al., 1994; Hwang et al., 1997). Transcriptional profiling of nitrate-regulated gene expression has extended knowledge of genes and processes regulated by nitrate availability and also identified a number of genes with distinct spatial and temporal patterns of expression (Ceres unpublished; Wang et al., 2000; Wang et al., 2003).
Nitrogen is most frequently the rate limiting mineral nutrient for crop production. Plants have evolved complex signaling and regulatory mechanisms to enable rapid physiological and metabolic response to changes in the supply of inorganic nitrogen in the soil. Part of this regulation is achieved through transcriptional regulation of gene expression. This is an important mechanism for allowing plants to adjust nitrogen uptake, reduction and transport in response to changing environmental conditions. Inefficiencies in nitrogen use efficiency may be overcome through the use of nitrogen regulated gene expression to modify the response of rate limiting enzymes and metabolic pathways to changes in nitrogen availability.
The ability to modify plant gene expression and ultimately the phenotype of a plant using nitrogen-inducible promoters can be a powerful method for deploying nitrogen transgene product concepts in the field. We have identified promoters that are induced in nitrogen starved Arabidopsis plants in response to nitrate provision as well as promoters that are induced by decreases in nitrate concentration.